Excited Thomas-Efimov levels in ultracold gases

TL;DR

This paper reinterprets experimental data on ultracold cesium gases, providing evidence that observed three-body resonances are the ground Efimov state, and discusses prospects for detecting excited Efimov states in other ultracold atomic systems.

Contribution

It offers an intuitive and numerical analysis showing that existing cesium experiments observe the ground Efimov state, clarifying previous interpretations and proposing methods to observe excited states.

Findings

Cesium experiments observe the ground Efimov state, not a separate Efimov state.

The observed cesium resonance is a Borromean molecular state.

Potential detection of excited Efimov states in rubidium and cesium at specific magnetic fields.

Abstract

Since the early days of quantum physics, the complex behavior of three interacting particles has been the subject of numerous experimental and theoretical studies. In a recent Letter to Nature, Kraemer et al. [Nature (London) 440, 315 (2006)] report on experimental ``evidence for Efimov quantum states'' in an ultracold gas of cesium atoms. Such quantum states refer to an infinite series of energy levels of three identical Bose particles, accumulating at the threshold for dissociation as the scattering length of each pair is tuned to infinity. Whereas the existence of a single Efimov state has been predicted for three helium atoms, earlier experimental studies concluded that this elusive state had not been found. In this paper we show by an intuitive argument and full numerical calculations that the helium and cesium experiments actually provide evidence of the same, ground state of this trimer spectrum, which the helium experimentalists and pioneering theoretical studies had not associated with Efimov's effect. Unlike the helium trimer, the observed 133Cs_3 resonance refers to a Borromean molecular state. We discuss how as yet unobserved, excited Efimov quantum states might be detected in ultracold gases of 85Rb and of 133Cs at magnetic field strengths in the vicinity of 0.08 T (800 G).